The present invention relates to an electron beam exposure apparatus for projecting an image formed by electron beams onto the object to be exposed such as a wafer via a reduction electron optical system.
In the mass production process for manufacturing semiconductor memory devices, an optical stepper with high productivity is used. However, in the manufacture of memory devices since 1G and 4G DRAMs having a line width of 0.2 .mu.m or less, an electron beam exposure method that can assure high resolution and high productivity is expected to replace optical exposure.
As a conventional electron beam exposure method, a single-beam Gaussian scheme and variable forming scheme are popular, and since these schemes have low productivity, they are used in only applications that use excellent resolving performance of electron beams, such as mask drawing, studies and developments of VLSIs, exposure of ASIC devices that are manufactured in small quantities, and the like.
As described above, how to improve the productivity is a serious problem upon application of the electron beam exposure method to the mass production process.
In recent years, as a method of solving the above-mentioned problem, a stepping scheme has been proposed. This scheme (FIG. 9) aims at improving the productivity of drawing by forming repetitive portions of a memory circuit pattern as cells in units of regions having a width of several .mu.m.
However, the maximum exposure region that can be simultaneously exposed by this scheme is as small as about several .mu.m, which is the same as that in the variable forming scheme, and in order to obtain a broader exposure region, a plurality of (two or three) deflectors must be arranged, and chromatic aberration, distortion, and the like caused by deflection must be removed using an MOL (movable objective lens system).
In order to improve the productivity, it is required to broaden the drawing region. However, the deflection amount that can assure a resolution of 0.2 .mu.m or less and a stitching precision of 20 to 30 nm is about several mm.
As described above, in the conventional electron beam exposure apparatus, the region that can be exposed simultaneously, i.e., a so-called exposure region is extremely smaller than that in an optical exposure apparatus or the like. For this reason, a full plate method of exposing the entire wafer by scanning an electron beam and mechanically scanning the wafer and mask is used. In order to expose the entire surface of the wafer, the stage must be reciprocally scanned a large number of times, and consequently, the stage scanning time becomes a main factor that determines productivity. Hence, a very long time is required for exposing a single wafer as compared to the optical exposure apparatus.
The throughput can only be greatly improved by either increasing the scanning speed or broadening the irradiation region. On the other hand, on a conventional irradiation region as small as several .mu.m, the image is blurred under the influence of the space charge of beam currents, if the current density increases. That is, since the maximum irradiation current value is limited, the problem still remains unsolved even when the high scanning speed is attained.
As described above, it is hard to broaden the exposure region as long as image formation is done using a narrow region in which the on-axis aberration of an electron optical system is small like the conventional exposure method.